1. Field of the Invention
This invention generally relates to semiconductor devices having a GaN-based semiconductor layers, and more particularly, a GaN-based semiconductor device, a fabrication method of the same, a substrate for fabricating the same, a fabrication method of the substrate, and a growth substrate of the same.
2. Description of the Related Art
The semiconductor device that employs a GaN-based semiconductor, particularly, gallium nitride (GaN) is used as a power device that operates at high frequencies and high power. High Electron Mobility Transistor (HEMT) is known as a semiconductor device that is suitable for amplification especially at high frequency ranges such as microwave, sub-millimeter wave, millimeter wave, and the like. The GaN-based semiconductor is made of, for example, GaN or a mixed crystal of GaN and AlN or GaN and InN.
With respect to HEMT that employs GaN, the technical development is in progress to realize the operation at a higher frequency and higher output. There are demands for increasing the electron density of 2-Dimensional Electron Gas (2DEG) and reducing the contact resistance between the source and drain electrodes and 2DEG (contact resistance of ohmic electrode), in order to realize the operation at a higher frequency and higher output. Also, an enhanced-mode HEMT (E mode HEMT) having a positive threshold voltage is being progressively developed to provide an amplifier that is operable by a single power supply.
HEMT has a normal HEMT structure and an inverted HEMT structure. In the normal HEMT structure, an electron supply layer is deposited on an electron traveling layer, and a gate electrode, a source electrode and a drain electrode are provided on the electron supply layer. In the inverted HEMT structure, the electron traveling layer is deposited on the electron supply layer, and the gate electrode, source electrode and, drain electrode are provided on the electron traveling layer.
Japanese Patent Application Publication No. 2003-229439 discloses an example of the normal HEMT structure (hereinafter, referred to as first conventional art). A GaN electron traveling layer (which corresponds to the channel layer in the first conventional art) to which impurities are not added is deposited above a sapphire substrate or SiC substrate via a buffer layer, an n-type AlGaN electron supply layer is deposited above the electron traveling layer via an AlGaN spacer layer, and the gate electrode, is formed on the electron supply layer. The source and drain electrodes are also provided on the electron supply layer via an n-type GaN contact layer.
Meanwhile, according to the first conventional art, the electric polarization is employed as a method for increasing the electron density of 2DEG, in addition to increasing the discontinuous energy of the band gap between the electron supply layer and the electron traveling layer. That is to say, if the positive charge is generated in the interface between the electron supply layer and the electron traveling layer due to the sum of self polarization and piezoelectric polarization, electrons will be induced to cancel the positive charge and the electron density of 2DEG will be increased. The first conventional art also describes that the composition ratio of AlN in the spacer layer is partially increased. Then, the positive charge resulting from the self-polarization and piezoelectric polarization is induced in the proximity of 2DEG to increase the electron density of 2DEG.
Japanese Patent Application Publication No. 2001-77353 discloses an example of the inverted HEMT structure in FIG. 1 (hereinafter, referred to as second conventional art). An AlGaN underlying layer is deposited on the (0001) plane of a sapphire substrate, and an n-type AlGaN electron supply layer having a composition ratio of AlN smaller than that of the underlying layer is deposited on the underlying layer. The electron traveling layer (which corresponds to an electron accumulation layer in the first conventional art) is deposited on the electron supply layer. The gate electrode, source electrode, and drain electrode are formed on the electron traveling layer. The 2DEG is provided in the proximity of the interface between the electron traveling layer and the electron supply layer. In the inverted HEMT structure, there is no AlGaN layer having a large band gap between the source and drain electrodes and 2DEG. The contact resistance of ohmic electrode can be decreased.
However, the normal HEMT structure as disclosed in the first conventional art essentially has the AlGaN electron supply layer having a large band gap between the source and drain electrodes and 2DEG. This causes a drawback of increasing the contact resistance of ohmic electrode.
The second conventional art has a drawback that the electron density of 2DEG is small. This may be caused as follows.
The electric polarization of a semiconductor layer includes self-polarization and piezoelectric polarization. The self-polarization is caused by a difference in electronegativity and is dependent on the type and orientation of crystal. The piezoelectric polarization is caused by distortion of crystal, when a thin film having a different lattice constant is deposited on a semiconductor layer. The charge induced in the interface between two semiconductor layers is the difference of the self-polarization and piezoelectric polarization in the two semiconductor layers. The density of 2DEG in the semiconductor layer equals to the sum of the self-polarization and piezoelectric polarization. For this reason, directions in which both self-polarization and piezoelectric polarization operate influence the density of 2DEG.
FIG. 1 schematically shows semiconductor layers below the gate electrode in the second conventional art. Here, the transverse direction of each layer schematically shows the magnitude of the lattice constant. The upward direction denotes the [0001] crystalline orientation of the wurtzite structure. An AlGaN underlying layer 22, an AlGaN electron supply layer 24, and a GaN electron traveling layer 26 are deposited on the (0001) plane of a sapphire substrate 20 in this order. The GaN crystal and AlGaN crystal are self-polarized so that the [000-1] orientation is positive. The direction of the self-polarization is determined by the direction of Ga—N bonding along the C-axis. Accordingly, in the [0001] crystalline orientation (Ga plane growth), self-polarizations PSP22, PSP24, PSP26 of the underlying layer 22, the electron supply layer 24, and the electron traveling layer 26 work in a downward direction (which is positive). Also, the AlN crystal has a larger self-polarization than that of the GaN crystal. The absolute value of the self-polarization is greater, as the composition ratio of AlN is greater. That is to say, the relationship shown below is satisfied.|PSP22|>|PSP24|>|PSP26|
The underlying layer 22 is provided so as to have a thickness enough for lattice relaxation, and the piezoelectric polarization hardly occurs in the underlying layer 22 in the vicinity of the electron supply layer 24. However, the piezoelectric polarization occurs in the electron supply layer 24 and the electron traveling layer 26 due to the difference in the lattice constant between these layers and the underlying layer 22. The electron supply layer 24 and the electron traveling layer 26 have larger lattice constants than that of the underlying layer 22. This results in the upward piezoelectric polarizations PPE24 and PPE26 in the electron supply layer 24 and the electron traveling layer 26. Also, the electron traveling layer 26 has a lattice constant greater than that of the electron supply layer 24 with respect to the underlying layer 22. That is to say, the relationship shown below is satisfied.|PPE24|<|PPE26|
In the [0001] crystalline orientation (Ga plane growth), if the electron traveling layer 26 is greater in lattice constant than a target (in the aforementioned case, the electron supply layer 24), the piezoelectric polarity works in a positive upward direction. The direction in which the piezoelectric polarization works is determined by (1) the direction of stress applied to the crystal and (2) the crystalline orientation (GaN plane or N plane). In the afore-mentioned case, the magnitude of the lattice constant described above and Ga plane growth cause the piezoelectric charge to work in the upward direction.
The difference between the sum of polarization in the electron supply layer 24 and that of polarization in the electron traveling layer 26 results in the charge generating in the interface between the electron supply layer 24 and the electron traveling layer 26 Accordingly, negative charge develops in the interface between the electron supply layer 24 and the electron traveling layer 26, causing the electron density of 2DEG to decrease. In this case, the polarizations of the electron supply layer 24 and the electron traveling layer 26 cancel each other, which decreases the polarized electrode. As a result, the electron density of 2DEG also decreases.
As described, it is possible to increase the electron density of 2DEG with the use of the self-polarization and piezoelectric polarization in the normal HEMT structure. However, the ohmic contact resistance is high. In contrast, the inverted HEMT structure has a decreased ohmic contact resistance. However, there is a problem that the electron density of 2DEG is low because of the self-polarization and piezoelectric polarization.
According to the first and second conventional arts, the AlGaN film or the GaN film is deposited on the sapphire substrate or the SiC substrate. When the AlGaN film or the GaN film is deposited, Ga is first deposited on the sapphire substrate or the SiC substrate. Therefore, it is not easy to deposit the AlGaN film or the GaN film in the [000-1] crystalline orientation.